1. Field of the Invention
The present invention relates to variable speed controls for AC induction motors such as may be used in air moving applications.
2. Discussion of Related Art
Many applications for electric motors demand variable speeds from the motor. For example a blower motor in a household heating, ventilation and air-conditioning (HVAC) system will typically be a fractional horsepower motor driving a blower unit or fan blade which requires motor speed adjustment to enable efficient operation of the HVAC system.
Inexpensive induction motors are desirably utilized in many applications. These motors are not particularly well adapted for variable speed usage. Rather they are designed to operate efficiently only at one best speed and inefficiencies result when trying to run the motor at other than the designed speed. However, many systems, such as the above HVAC applications, would benefit greatly from having a wider range of motor speeds available.
In the past art, a variable range of speeds from one induction motor was obtained through the use of expensive controllers changing the frequency and voltage of the input to the motor windings. Expensive controllers such as these were necessary because, as the input to the motor windings strays farther from sinusoidal, motor efficiency and power factor drop while current and total harmonic distortion rises, resulting in unacceptable noise, heat, efficiency loss, and motor life.
FIGS. 1A and 1B are diagrams showing the electrical hookups of standard single speed and multi-tap PSC motors, respectively. The known multi-tap technique involves using a multi-tap motor to attain a number of fixed selectable speeds by mechanical switching between the taps. A multi-tap motor may have connections physically switched between terminals to achieve multiple, but not variable, speeds from its AC induction motor. FIG. 2 shows how a standard single triac control is typically installed to operate a PSC motor, as known in the art. The drawbacks of the motors of FIGS. 1A and 1B within a continuously variable speed motor environment are self evident. The known triac control of FIG. 2 does not permit optimum performance in terms of current draw and total harmonic distortion (THD) since the line voltage to both the auxiliary and main coils is chopped by the triac. Thus, known motor controllers utilizing inexpensive switching mechanisms, such as triacs, to control power to the motor windings by “chopping” the sinusoidal waveform input were thought to be of limited use in applications of continuously variable motor speed control.
Further, in the known art, some control schemes demand that the controller characteristics be matched to the known induction motor type characteristics, thereby limiting the availability of retrofitting a single type of variable speed controller to the large installed base of induction motors. Some known motor systems include a motor/controller combination that utilizes RPM and torque sensing of a motor/blower combination to infer airflow. This technique, however, dictates that the motor/controller/blower combination must be tested together so to determine the speed, torque, and airflow relationships. Once these relationships have been tested and certain parameters have been determined, a lookup table for the controller can be replicated in a microprocessor.
The disadvantage of this method is that each motor/controller/blower combination must be factory programmed. Two systems requiring the same size motor may differ only because of the unique controller software. In the event of a motor/controller/blower failure it may prove difficult to obtain a direct replacement.
It can thus be understood that there remains a need in the art for a variable speed controller for induction motors which is easily retrofitted to existing motors, provides efficient operation of the motor at variable speeds and is inexpensive in comparison to the controllers of the known art.